The kidneys fulfil many functions, including the removal of water, the excretion of catabolites (or waste from the metabolism, for example urea and creatinine), the regulation of the concentration of the electrolytes in the blood (sodium, potassium, magnesium, calcium, bicarbonates, phosphates, chlorides) and the regulation of the acid/base equilibrium within the body, which is obtained in particular by the removal of weak acids (phosphates, monosodium acids) and by the production of ammonium salts.
In individuals who have lost the use of their kidneys, since these excretion and regulation mechanisms no longer work, the body accumulates water and waste from the metabolism and exhibits an excess of electrolytes (in particular sodium), as well as, in general, acidosis, the pH of the blood plasma shifting towards 7 (the blood pH normally varies within narrow limits of between 7.35 and 7.45).
In order to overcome renal dysfunction, resort is conventionally made to a blood treatment involving extracorporeal circulation through an exchanger having a semipermeable membrane (haemodialyser) in which the patient's blood is circulated on one side of the membrane and a dialysis liquid, comprising the main electrolytes of the blood in concentrations close to those in the blood of a healthy subject, is circulated on the other side.
Furthermore, a pressure difference is created between the two compartments of the haemodialyser which are delimited by the semipermeable membrane, so that a fraction of the plasma fluid passes by ultrafiltration through the membrane into the compartment for the dialysis liquid.
The blood treatment which takes place in a haemodialyser as regards waste from the metabolism and electrolytes results from two mechanisms of molecular transport through the membrane.
On the one hand, the molecules migrate from the liquid where their concentration is higher to the liquid where their concentration is lower. This is diffusive transport or dialysis.
On the other hand, certain catabolites and certain electrolytes are entrained by the plasma fluid which filters through the membrane under the effect of the pressure difference created between the two compartments of the exchanger. This is convective transport.
Three of the abovementioned functions of the kidney, namely the removal of water, the excretion of catabolites and the regulation of the electrolytic concentration of the blood, are therefore performed in a conventional blood treatment device by the combination of dialysis and blood filtration (this combination is referred to as haemodialysis).
As regards the regulation of the acid/base equilibrium inside the body, the approach adopted to overcome renal deficiency is to act on a mechanism by which the acid/base equilibrium inside the body is regulated, this mechanism consisting of the buffer systems of the blood, the main one of which comprises carbonic acid, as a weak acid, associated with its alkali salt, bicarbonate. This is why, in order to correct acidosis in a patient suffering from renal insufficiency, he is administered with bicarbonate via the vascular route, directly or indirectly, during a haemodialysis session.
The administration is indirect when the bicarbonate is involved in the composition of the dialysis liquid and passes into the blood by diffusion.
One drawback of this method is connected with the fact that bicarbonate precipitates with calcium and magnesium, which belong to the conventional components of a dialysis liquid.
In order to limit this reaction, an acid (acetic acid) is added to the dialysis liquid in order to lower its pH, which has the side effect of increasing the partial pressure of carbon dioxide in it, and has the undesirable consequence of inducing in the patient the discomfort resulting from the excess of this gas in his blood. Furthermore, in view of the maximum acceptable concentration of acid in the dialysis liquid, calcium deposits are produced in the circuits of the dialysis machine, and these need to be removed.
The administration of the bicarbonate is direct when the dialysis liquid is free of bicarbonate and the patient is infused with a sodium bicarbonate solution. This method has the benefit that it avoids having to combine, in the same treatment liquid, the substances which precipitate in the absence of acid.
However, it poses the problem of regulating the sodium concentration in the dialysis liquid so that the patient's body tends towards a determined sodium concentration. This is because, when the patient is infused with sodium bicarbonate with a view to reaching a determined bicarbonate concentration inside the patient's body, an amount of sodium is introduced therein which has not to date been taken into consideration, or has been taken into consideration only empirically, when the sodium concentration of the dialysis liquid is fixed.
In other words, with the above cited existing systems used for implementing the method which has just been described, no provision is made for regulating both the infusion flow rate of the sodium bicarbonate solution and the sodium concentration of the dialysis liquid such that the patient's body tends towards a precise predetermined concentration both of bicarbonate and of sodium.
To solve the above mentioned drawbacks, the prior art comprises dialysis apparatus wherein the dialysate conductivity is controlled in order to reach a desired post-dialysis plasmatic conductivity, i.e. conductivity (or sodium concentration) of the patient's blood at the end of the dialysis treatment.
It is known, for example from EP 1389475, a dialysis apparatus provided with a conductivity system that computes the dialysate conductivity (corresponding to the dialysate sodium concentration) from periodic measurements of the sodium blood concentration allowing the sodium level of the patient to reach a prescribed end-of-session value.
This dialysis apparatus includes a device for preparing a dialysis liquid containing sodium having a pump for regulating the sodium concentration and a dialysis liquid circuit with a feed line and a discharge line; the feed line has one end connected to the device for preparing a dialysis liquid and another end connected to a dialyser.
The discharge line has, as well, an end connected to the dialyser, it is also provided a bag and a pump for infusing a patient with an infusion solution containing sodium at a determined and known concentration.
A structure for determining the sodium concentration [Na+]dial of the dialysis liquid is also provided so that the patient's body tends towards a desired sodium concentration [Na+]des, as a function of the dialysance D for sodium of the dialyser, of the desired sodium concentration [Na+]des inside the patient's body, of the infusion flow rate and of the sodium concentration [Na+]sol of the infusion solution.
A control unit drives the pump for regulating the sodium concentration of the dialysis liquid such that this concentration is equal (tends towards) to the determined concentration [Na+]dial.
One of the problems of the dialysis apparatus of the prior art is presently the choice of the appropriate post-dialysis plasmatic conductivity target.